In-fiber Mach-Zehnder Interferometer Research Articles

A fiber-optic refractometer for humidity measurements using an in-fiber Mach–Zehnder interferometer

An in-fiber Mach–Zehnder interferometer (MZI) was proposed and experimentally demonstrated for refractive index (RI) and relative humidity measurements. The sensor is composed of a hollow-core fiber (HCF) sandwiched between two single-mode fibers (SMFs) with a length of 8mm. A well-defined interference pattern was obtained as a result of fiber–core mismatch and core–cladding mode interference. The selected interference dip was extremely sensitive to the ambient RI and provided a stable RI response with sensitivities of up to 155.6nm/RIU and 163.1dB/RIU for water-based solutions close to the RI of 1.33, which makes it capable of environment humidity measurements with a sensitivity of 1.29pm%−1 and 0.00542dB%−1.

Effect of waist diameter and twist on tapered asymmetrical dual-core fiber MZI filter.

A compact in-fiber Mach-Zehnder interferometer (MZI) filter fabricated from custom-designed asymmetrical dual-core fiber is numerically analyzed in detail and experimentally verified. The asymmetrical dual-core fiber has core diameters and a core pitch of 6.9, 6, and 19.9 μm, respectively. The fiber tapering technique is introduced to fuse the originally uncoupled cores into strong coupling tapered regions. The length and diameter of the waist region have a close impact on the splitting ratio, which further affects the spectral properties of the MZI filter. The field evolution with varied waist parameters is characterized by the finite element method and beam propagation method. Repeatable comb filters with ∼15 dB extinction ratio are successfully achieved under the guidance of simulated optimum conditions. The twist-induced circular birefringence gives rise to a retardance that causes the spectral shifts of the MZI filter. The theoretical and experimental results confirm that the relative wavelength shift is proportional to the retardance, which follows a sinc function in the limit of a large twist rate.

Fiber Sensor Based on Interferometer and Bragg Grating for Multiparameter Detection

An integrated in-fiber Mach-Zehnder interferometer (MZI) and fiber Bragg grating (FBG) sensor that can simultaneously detect localized temperature, strain, and ultrasonics is proposed and demonstrated. Using a 60-mm single-mode fiber with a 12-mm FBG spliced to two 3-mm multimode fibers and lead-in and lead-out single-mode fibers, the sensor can discriminate temperature and strain from the responses of the MZI and the FBG to the two static parameters. Ultrasonic signals are directly recorded as high-frequency voltage vibration by demodulation of Bragg wavelength shift using a corresponding sensing system. The overall system shows high practical potential for use in nondestructive testing.

In-Fiber Mach—Zehnder Interferometer Based on Waist-Enlarged Taper and Core-Mismatching for Strain Sensing**Supported by the National Natural Science Foundation of China under Grant Nos 61077006, 60727004, and 61077060, the China National Petroleum Corporation Science and Technology Development Projects under Grant No 2014B-4012, and the Science Research Plan Projects of Shaanxi Education Department under Grant No 14JK1580.

An in-fiber Mach—Zehnder interferometer for strain measurement is proposed and experimentally demonstrated. The sensor consists of a taper followed by a short section of a multi-mode fiber (MMF) and a dispersion compensating fiber (DCF), which is sandwiched between two single mode fibers (SMFs). The taper is used as a fiber coupler to excite cladding modes in the SMF, and these cladding modes transmit within the MMF and the DCF. The core mode and the cladding modes interfere in the DCF—SMF fusion point to form intermodal interference. A well-defined interference spectrum is obtained in the experiment. Selected interference dips are used to measure the strain changes. The experimental results show that this device is sensitive to strain with the wavelength-referenced sensitivity of 2.6 pm/μϵ and the power-referenced sensitivity of 0.0027 dB/μϵ, respectively.

TCF-MMF-TCF fiber structure based interferometer for refractive index sensing

A liquid refractive index (RI) sensor based on in-fiber Mach–Zehnder interferometer (MZI) by sandwiching multi-mode fiber (MMF) between two short sections of thinned core fiber (TCF) is proposed in this paper. The first section of TCF excites the high-order modes and the second section TCF couples the core mode and high-order modes into lead-out SMF to form intermodal interference. The sensor with MMF length of 20mm and TCFs length of 1mm was fabricated. The transmission spectrum variation of the sensor with respect to surrounding refractive index (SRI) has been studied by experiment. The results show that the central wavelength of dips/peaks shifting had a good linearity with SRI. The RI sensitivity of the sensor is 130.00nm/RIU over the RI range of 1.3333–1.4182. The RI sensitivity increase to 433.60nm/RIU after etching the MMF cladding of the sensor. The sensor keeps low dependence on temperature before and after etching.

A thermally annealed Mach-Zehnder interferometer for high temperature measurement.

An in-fiber Mach-Zehnder interferometer (MZI) for high temperature measurement is proposed and experimentally demonstrated. The device is constructed of a piece of thin-core fiber (TCF) sandwiched between two short sections of multimode fiber (MMF), i.e., a MMF-TCF-MMF structure. A well-defined interference spectrum is obtained owing to the core-mismatch, and the interference dips are sensitive to the ambient temperature. The experimental results show that the proposed interferometer is capable of high temperature measurement up to 875 °C with a sensitivity of 92 pm/°C over repeated measurements. The explored wavelength drop point may limit the measurement range, which can be improved by repeated thermal annealing.

In-fiber Mach–Zehnder interferometer based on multi-mode fiber and up-taper for curvature sensing

A new type of curvature sensor comprises a stub of multi-mode fiber and an up-taper is proposed and demonstrated experimentally. The whole fabrication process is quite simple and the sensor head is cost effective. Measurement results show that it has a maximum curvature sensitivity of −61.877nm/m−1 at 1.1718m−1 (the highest value of reported papers among in-fiber Mach–Zehnder interferometers) and −9.2115nm/m−1 from 0.865m−1 to 1.1172m−1. Temperature sensitivity of 89.01pm/°C within the range of 20–80°C has also been achieved, which implies the possibility for measurement of temperature.

Optical Fiber Refractometer Based on Etched-Stress Applying Parts PANDA Fiber

In this letter, an optical fiber refractometer fabricated by etched-stress applying parts (SAPs) PANDA fiber is proposed and experimentally demonstrated. The proposed structure depends on an in-fiber Mach-Zehnder interferometer (MZI), which is induced by the two orthogonal polarization modes of the fiber and the dip wavelength shifts as a function of the surrounding medium refractive index. In the sensor area, one of the axes is quickly etched to near the fiber core modifying the evanescent field, which results in increasing birefringence variation by the surrounding medium. The fiber has been chemically etched with the etched length of 5 cm. High sensitivity of 151 nm/RIU (refractive index unit) within the linear sensing range from 1.3401 to 1.3997 and a resolution of \(1.32 \times 10^{-4}\) RIU are obtained for the proposed liquid refractive index sensor.

Semi-open cavity in-fiber Mach-Zehnder interferometer for temperature measurement with ultra-high sensitivity.

We demonstrate a semi-open cavity in-fiber Mach-Zehnder interferometer based on optical fiber tube (OFT) for temperature measurement with high sensitivity. The interferometer is composed of an OFT sandwiched between two multimode fibers, with lateral offset. The air hole of the OFT was not completely sealed and liquid is poured into the air hole through the unsealed gap. Light from the multimode fiber is split into two beams: one beam transmits directly through the silica tube while the other travels along the liquid-filled cavity. The device has ultra-high temperature sensitivity due to the much larger thermo-optic coefficient of the liquid compared with that of silica. Experimental results show that the temperature sensitivity is 6.35nm/°C for an ethanol-filled structure.

In-fiber modal interferometer formed by offset-splicing in air-core photonic bandgap fiber

We present a compact in-fiber Mach–Zehnder interferometer (MZI) formed by an offset splicing technique in air-core photonic bandgap fiber (PBF). The MZI is implemented by using two cascaded offset-splicing joints which act as beam splitter and combiner. The influences of lateral offset amount and interferometer length on the performance of MZIs were investigated experimentally. Several in-fiber MZIs with different interferometer lengths were built and it is found that the interference dip wavelength spacing is inversely proportional to the interferometer length. The MZI relies on the interference occurring between the fundamental core mode (FCM) and a surface mode (SM) of the PBF. The potential applications of the proposed in-fiber MZI were further investigated as temperature and strain sensors. The temperature and longitudinal strain sensitivities of the MZI were measured to be 211.89pm/(°Cm) and −0.6pm/με, respectively.

Fast analyses and designs of long-period fiber grating devices with cosine-class apodizations by using Fourier mode coupling theory

This paper presents an analytical approach to fast analyzing and designing long-period fiber grating (LPFG) devices with cosine-class apodizations by using the Fourier mode coupling (FMC) theory. The LPFG devices include LPFGs, LPFG-based in-fiber Mach-Zehnder and Michelson interferometers, which are apodized with the cosine-class windows of cosine, raised-cosine, Hamming, and Blackman. The analytic models (AMs) of the apodized LPFG devices are derived from the FMC theory, which are compared with the preferred transfer matrix (TM) method to confirm their efficiencies and accuracies. The AM-based analyses are achieved and verified to be accurate and efficient enough. The AM-based analysis efficiency is improved over 1318 times versus the TM-based one. Based on the analytic models, an analytic design algorithm is proposed and then applied to designing these LPFG devices, which has the complexity of O(N) and is far faster than the existing design methods.

Ultrasensitive Refractive Index Sensor Based on Microfiber-Assisted U-Shape Cavity

A novel in-fiber Mach-Zehnder interferometer formed by a microfiber-assisted U-shape cavity (MFA-USC) with ultrahigh refractive index (RI) sensitivity and an ultra-wide RI measurement range is demonstrated. The MFA-USC is fabricated by splicing a section of micro-fiber (diameter is ~ 63 μm, length is 688 μm) tapered from single-mode fiber (SMF) between two sections of SMF with designed fiber geometry relationship. An ultrahigh sensitivity (higher than 126 000 nm/RIU at a RI of 1.445) within a RI range 1.320-1.446 and 1.462-1.738 has been experimentally achieved. This sensitivity is the highest ever reported to date to the best of our knowledge.

A Mach–Zehnder interferometric humidity sensor based on waist-enlarged tapers

A novel humidity sensor based on an in-fiber Mach–Zehnder interferometer (MZI) is proposed and demonstrated. The sensor head is formed by a single-mode–multimode–single-mode (SM–MM–SM) fiber structure through arc fusion splicing. The intermodal interference is achieved by two waist-enlarged fiber tapers at the coupling points of the multimode fiber and single-mode fibers. The sensor has a linear response to humidity with enhanced sensitivity of −0.119dB/%RH in the range of 35–90%RH. Additionally, the sensor exhibited excellent temperature immunity in the temperature test. Such easily fabricated, cost-effective and temperature-immune fiber interferometer could be used for high sensitivity humidity sensing applications.

A strain sensor based on in-line fiber Mach–Zehnder interferometer in twin-core photonic crystal fiber

We experimentally demonstrate a strain sensor realized using a twin-core photonic crystal fiber (PCF). The strain sensor element consists of a 10cm long in-fiber Mach–Zehnder interferometer consisting of twin-core PCF. The output spectra of the sensing element is measured and analyzed under various strain levels. The sensitivity of the strain measurement of −0.31pm/µε is achieved within a range from 0 to 4000µε. The effect of temperature is also analyzed.

Highly Sensitive Curvature Sensor Using an In-Fiber Mach–Zehnder Interferometer

A highly sensitive curvature sensor using an in-fiber Mach–Zehnder interferometer is proposed and experimentally demonstrated. The sensor has a simple configuration—a short section of single-mode fiber is sandwiched between two short sections of multimode fibers to achieve a compact in-fiber Mach–Zehnder interferometer. The wavelength shift of selected dip in the Mach–Zehnder interference spectrum responds linearly with curvature and shows a high sensitivity up to ${-}{\rm 14.40}{\pm}{\rm 0.02}~{\rm nm}^{\ast}{\rm m}$ .

An In-Fiber Mach–Zehnder Interferometer Based on Arc-Induced Tapers for High Sensitivity Humidity Sensing

A new humidity sensor based on in-fiber Mach-Zehnder interferometer (MZI) is proposed and demonstrated. The interferometer consists of two arc-induced ellipsoid fiber tapers and a section of single mode fiber between them. In our humidity experiment, the relative humidity sensitivity of MZI is -0.047 nm%RH in the range of 50%-90%RH. Temperature response characteristics of this MZI are also studied, and the temperature sensitivity is 0.067 nm/°C from 27 °C to 120°C. The experimental results show that the sensor has a linear response to humidity and temperature with linear fitting of 0.993 and 0.992, respectively. The MZI presented in this paper has advantages of low cost, easy fabrication, and high sensitivity, which can be a potential detector for the relative humidity measurement.

Simultaneous measurement of relative humidity and temperature with PCF-MZI cascaded by fiber Bragg grating

Simultaneous measurement of relative humidity and temperature has been realized by using an optical fiber sensor formed by cascading a photonic crystal fiber (PCF)-based in-fiber Mach-Zehnder interferometer (MZI) and a fiber Bragg grating (FBG). The PCF-MZI was fabricated by using a short PCF fusion-spliced between two single-mode fibers with its air holes in the cladding area being collapsed in the splicing regions. It was then coated with a layer of polyvinyl alcohol (PVA), whose refractive index is sensitive to humidity. Because the PCF-MZI and FBG have different responses to humidity and temperature, simultaneous measurement has been achieved with resolutions of 0.13% RH and 1.0°C for humidity and temperature, respectively. The relative humidity measurement range is up to 30%–95% RH.

In-fiber Mach-Zehnder interferometer based on fiber core etched air-bubble and core diameter mismatch for liquid refractive index sensing

A kind of optical fiber liquid refractive index sensor is proposed based on fiber core etched air-bubble and core diameter mismatched in-fiber Mach-Zehnder interferometer. A core etched standard single-mode fiber is spliced to another core etched standard single-mode fiber (SMF) to form an air bubble at the connecting point, and a 20 mm thinned fiber is cascaded (TCF) by fusion splicing method, which is 20 mm apart from the bubble, then a leading-mode fiber is spliced to the thinned fiber to form a structure of SMF-(air-bubble)-SMF-TCF-SMF in-fiber Mach-Zehnder interferometer liquid refractive index sensor. The air-bubble and the two fiber core diameter mismatched points serve as optical couplers for modes conversion. The transmission spectrum of sensor is studied by experiment. Results shows that the peak power changes with respect to surrounding refractive index with good linearity. The sensitivity of the sensor is 142.537 dB/RIU in the range of 1.3511.402 with linearity of 0.996, making it a good candidate for bio-chemical measurements.

Highly Sensitive In-Fiber Refractive Index Sensor Based on Down-Bitaper Seeded Up-Bitaper Pair

We report a novel highly sensitive in-fiber refractive index (RI) sensor based on the Mach-Zehnder interferometer (MZI). This sensor is composed of an up-fusion-bitaper (UFBT) pair and an embedded down-stretching-bitaper (DSBT), where the UFBT excited high-order cladding modes and the DSBT enhanced the evanescent field. By employing the interaction between the strong evanescent field of high-order cladding mode and ambient environment, an RI sensitivity of 86.565 nm/cm/RIU is achieved in the RI range from 1.3332 to 1.4140. This sensitivity is about an order of magnitude higher than that of the conventional and taper-based improved in-fiber MZIs.

High-Sensitive Fiber-Optic Refractometer Based on a Core-Diameter-Mismatch Mach–Zehnder Interferometer

An in-fiber Mach-Zehnder interferometer (MZI) for high-sensitive refractive index (RI) measurement is proposed and experimentally demonstrated. The sensing head is formed by a single-mode fiber (SMF) sandwiched between two short sections of thin-core fibers (TCFs), i.e., TCF-SMF-TCF MZI configuration with a length of 9 mm. A well-defined interference pattern is obtained as the result of fiber-core mismatch and core-cladding modes interference. The selected interference dip is extremely sensitive to ambient RI, providing a stable RI response with sensitivity up to 159 nm/RIU for water-based solutions close to the RI of 1.33, making it a good candidate for bio-chemical measurements.